Umbrella base



FIG. 1 is a perspective view of an umbrella base showing my new design;

FIG. 2 is a front view of the design of FIG. 1;

FIG. 3 is a rear view of the design of FIG. 1;

FIG. 4 is a left side view of the design of FIG. 1;

FIG. 5 is a right side view of the design of FIG. 1;

FIG. 6 is a top view of the design of FIG. 1; and,

FIG. 7 is a bottom view of the design of FIG. 1.

The broken lines depict portions of the article that form no part of the claimed design. 

CLAIM The ornamental design for an umbrella base, as shown and described. 